Microscopic particles such as particular species of bacteria lying in a fluid such as water or air, can be identified by detecting their pattern of light scatter when they pass through a light beam such as a laser beam. A plurality of photodetectors can be positioned to detect light scattered in different directions from a small detect zone lying along the laser beam. The outputs of the photodetectors are delivered to a computer that compares the pattern of light scatter for an unknown particle that is passing through the detect zone, to the patterns of a list of known species of particles, usually microorganisms, to determine whether the unknown particle is a member of one of the listed species.
Previously, applicant custom mounted the multiple photodetectors on a frame that could position photodetectors to detect light scattered in different directions and at different angles from the forward direction of the laser beam. Such a frame and detectors can be awkward and expensive to build and connect to. Furthermore, such a setup can result in a rat""s nest of wires extending from the multiple photodetectors to a cable leading to the computer. The large number of custom terminated wires can result in reduced reliability of electrical connections and considerable signal losses along some of the wires. A system that avoided the need for such difficult mounting and such rat""s nest of wires, would be of value.
In accordance with one embodiment of the present invention, an apparatus and method are provided that improve operation of a system for identifying microscopic particles in a fluid by directing a laser beam through the fluid and detecting light scattered in multiple directions by a particle passing through the detect zone. Instead of mounting individual detectors on a frame, so each detector can detect light scattered in a predetermined direction from the detect zone, and providing multiple wires leading from each detector to an amplifier that connects to a computer, applicant provides a holographic optical element and at least one linear array of photodetectors such as CCDs (charge coupled devices). The holographic optical element is constructed with multiple areas, or sections, that are each constructed to direct light received from the direction of the detect zone, toward a selected one of the photodetectors of the linear array. A converging lens preferably lies immediately forward or rearward of the holographic optical element. Only a single holographic optical element is required to intercept light scattered in multiple directions within a wide angle from the detect zone. By directing the scattered light to selected photodetectors of a linear array, applicant can use available linear arrays of photodetectors such as CCDs. The linear array not only simplifies mounting of photodetectors and avoids multiple custom connections and a rat""s nest of wire which all degrade performance, but enables rapid readout.
The holographic optical element can be constructed with sections that each intercept light scattered in a particular circumferential direction and at a particular angle to the forward direction of the laser beam, to mimic the detection of light by individually mounted photodetectors of applicant""s prior systems. In another arrangement, the holographic optical element takes advantage of the fact that the element can direct light from an area of any shape on the hologram to a selected photodetector, to make detections that facilitate the identification of the unknown particle. In one arrangement, the holographic optical element is divided into sections that are each in the shape of a narrow ring. In another arrangement, one part of the hologram forms sections that are parts of rings, while another part forms multiple pie-shaped sections. In another arrangement, the holographic optical element is divided into multiple small sections, or areas that each direct intercepted light to a different photodetector. Substantially all locations in each small area lie within about 5xc2x0 of its center. The photodetector outputs can be combined to simulate rings, pie-shaped sections, etc.
A linear array of CCDs forms an image scanner, to deliver the outputs of the detectors of the array sequentially to the computer. This simplifies connection of the linear array to the computer, especially if a large number of detectors of the linear array are used.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.